Covers for roll cores are used in demanding industrial environments such as paper mills and are subjected to high temperatures, corrosive chemicals and dynamic loads. In a typical paper mill, large numbers of covered rolls are utilized for transporting web sheets which become paper as well as for processing these web sheets into finished paper. It is essential that these covered rolls be precisely balanced and include surfaces that maintain specific configurations and tight tolerances. Calendering is a process utilized within a paper mill for improving the smoothness, gloss, printability and thickness of paper. Covered rolls utilized in these calendering processes, usually referred to as calender rolls, super-calendar rolls or soft nip calendar rolls are subjected to high dynamic loads. As previously mentioned, a calender roll actually contributes to the processing of the paper rather than merely transporting the web through the paper mill machinery. In order to function properly, a calendar roll must have a surface of a predetermined hardness of a high degree.
Several methods have been taught in the prior art for alleviating residual stresses that may develop during the heating and curing steps performed during the fabrication of covered roll cores. Residual stresses can develop for many reasons. For example, such stresses can result from a mismatch in the thermal expansion properties between the materials utilized in the cover and the material utilized in the roll core when these materials are bonded together.
Several methods which discuss reducing residual stresses in a covered roll are disclosed in U.S. Pat. Nos. 5,601,920 and 5,958,533, (both issued to Paasonen et al. and hereinafter collectively referred to as xe2x80x9cthe Paasonen patentsxe2x80x9d). Under the Paasonen patents, a compressive layer formed of a three-dimensional spacer fabric is included between a metallic roll core and a covering layer. The three-dimensional spacer fabric is formed of a top surface, a bottom surface and a void space or gap therebetween. The spacer fabric may be formed of suitable fibrous materials such as polyester, DACRON(copyright), NYLON(copyright), or fiberglass. Under the method disclosed in the Paasonen patents, the compressive layer formed of the spacer fabric is first applied to the metallic roll core. Next, the covering layer is placed over the compressive layer and allowed to fully cured. The void space or gap within the compressive layer is arranged to change in volume in response to volume changes in the covering layer which occur during curing to avoid the buildup of residual stresses. After the covering layer has been allowed to cure, a relatively high viscosity thermoset polymer is injected, by positive pressure, through holes drilled through the covering layer and into the void space within the compressive layer which upon curing is said to add strength to the resulting covered roll.
While the aforementioned compressive layer disclosed in the Paasonen patents may provide some degree of residual stress reduction, there are several drawbacks to the construction of the compressive layer of the Paasonen patents that could be surmounted to increase the overall strength of the resulting covered roll and increase the adhesion between the covering layer and the metal roll core. First, while the void space or gap of the compressive layer may make possible the injection of certain highly viscous thermoset resins therein where those resins are injected by positive pressure, the large portion of the injected thermoset will cure within the void space defined within the spacer fabric rather than bind with the fibrous material forming the spacer fabric. Only a small portion of the injected thermoset material will actually bind with the spacer fabric. The larger unbound portion of thermoset resin will form a weak brittle mass that will do little to add to the overall strength of the resulting covered roll and will do little to serve to increase the adhesion between the metal roll core and the covering layer. Secondly, the fibers of the compressive layer disclosed in the Paasonen patents are not oriented or aligned in any manner that would increase the overall strength of the covered roll. By orienting long continuous fibers in directions parallel and perpendicular to the longitudinal axis of the metal roll core, a covered roll having increased strength may be realized. Moreover, by adjusting the orientation of these longitudinally and axially oriented fibers, the strength of the resulting covered roll may be tailored to suit a customer""s requirements for different industrial applications. Also, the inclusion of such oriented long continuous fibers increases adhesion between the covering layer and the metal roll core.
Under the present invention, the covered roll core comprises a roll core base, an under-layer formed of a densely packed fiber under-layer and a covering layer circumferentially surrounding the under-layer. After the covering layer has been applied to the under-layer, it is allowed to cool from its cure temperature. After cooling, the under-layer, formed of densely packed fibers is infused with a thermoset resin by use of vacuum pressure. Due to the dense packing of fibers in the under-layer; most of the infused thermoset resin will bind with the fibers to form a stronger covered roll. This construction will also result in greater adhesion between the covering material and the metal roll core. Also, because a larger amount of infused thermoset resin will be bound up with the densely packed fibers of the under-layer, a lower viscosity thermoset resin may be employed to maximize the fiber-resin ratio in the resulting composite. The densely packed fiber under-layer will also provide added strength and greater adhesion because it is comprised of a plurality of directionally oriented fibers. Any suitable covering material may be employed in combination with the densely packed fiber mat under-layer.
In a variation of the first embodiment, the covering layer comprises a helically wound strip of rubber.
In another variation of the first embodiment, the covering layer comprises at least one sheet of rubber.
In another variation of the first embodiment, the covering layer comprises a plurality of rubber sheets.
In another variation of the first embodiment, the covering layer comprises a helically wound fabric strip impregnated with a thermoset resin.
In another variation of the first embodiment, the covering layer comprises urethane.
In another variation of the first embodiment, the covering layer is applied to the densely packed fiber mat under-layer by casting.
In another variation of the first embodiment, the covering layer comprises a strip of urethane.
In another variation of the present invention, a vacuum system is utilized for infusing thermoset resin into the densely packed fibers forming the under-layer.